Radar antenna mount

ABSTRACT

In a short range radar detection device for burglar and like intrusion protection, operating on the doppler principle of detecting a frequency shift caused by a moving object, the mounting of the antenna provides structural rigidity and at the same time automatically furnishes the capacitances between the antenna, ground and an inductor, that are required for tuning the antenna system.

United States Patent Pronovost 1 Aug. 8, 1972 [54] RADAR ANTENNA MOUNT2,636,122 4/1953 Hayes ..343/747 [72] Inventor: Joseph L Pronovost 483,237,191 2/1966 BOjkO ..343/5 PD Birchbank Rd., Bramalea, Ontario,Canada Primary ExammerSamuel Felnberg Attorney-Peter Kirby, Charles P.Curphey and Norris [22] Filed: Oct. 12, 1970 M. Eades 1 l. .1 ,7 [2 1APP 79 91 57 ABSTRACT [52] U S Cl 343/5 PD 343/702 343/747 In a shortrange radar detection device for burglar and 343/805 343/906; likeintrusion protection, operating on the doppler Int Cl 9/00 principle ofdetecting a frequency shift caused by a Fie'ld 805 906 moving object,the mounting of the antenna provides [56] References Cited UNITED STATESPATENTS 2,269,947 l/1942 Ludwig ..343/906 structural rigidity and at thesame time automatically furnishes the capacitances between the antenna,ground and an inductor, that are required for tuning the antenna system.

6 Claims, 6 Drawing Figures PATENTEU 3 3,683, 376

sum 2 or 3 Ill IIIIIIIIII H F5 7 I PATENTEDAUB 8 m2 3.683.376 SHEET 3[IF 3 RADAR ANTENNA MOUNT This invention relates to a manner of mountingan antenna in a radar device.

More specifically, it relates to a manner of mounting an antenna inwhich the structural elements serve simultaneously to provide thecapacitances between the antenna, ground and an inductor that arerequired for tuning the antenna system.

In the description that follows, the invention is exemplified as appliedto a short range radar detection device for detecting the movementofobjects in its vicinity. The device operates on the doppler principle ofdetecting signals returned by moving objects at a slightly differentfrequency from that of the transmitted signal. Stationary objects arenot detected.

While the device can be employed for any type of surveillance duty, theparticular purpose for which it has been developed is as an alarm devicefor the detection of burglars or other intruders into the area undersurveillance, e.g. a shop, warehouse or private home.

Devices of this general type are already known, but they suffer from thedisadvantage that they are complex and expensive, and consequentlybeyond the financial reach of many persons, especially private citizens.

An object of the present invention is to provide an antenna mount thatwill render a device of this character so simple in construction that itcan be mass produced and sold at a price well within a range that wouldwillingly be paid by individual house and apartment dwellers in returnfor improved security, e.g. a cost of up to about $100.

A further object of the invention is to achieve a device that can bemade for this low price without sacrifice of efficiency and reliabilityof operation.

Further objects and features of the invention will become apparent fromthe following description which, together with the accompanyingdrawings, illustrates one embodiment of the invention.

While the antenna mount is herein shown and described in associationwith a specific form of radar device, it is to be understood that themount may be employed with devices that differ in other aspects of theircircuits and constructions. Thus the illustrated embodiment is shown byway of example only, and not by way of limitation of the broad scope ofthe invention, which latter is defined in the appended claims.

In the drawings:

FIG. 1 is a small scale, perspective view of a radar device;

FIG. 2 is the electrical circuit of the device of FIG. 1, showndiagrammatically;

FIG. 3 is a larger scale, broken away side view of an antenna mountemployed in the device of FIG. 1;

FIG. 4 is a section on lV-lV in FIG. 3;

FIG. 5 is a further enlarged fragment of FIG. 3; and

FIG 6 is an exploded version of FIG. 5.

As shown in FIG. 2, an oscillator 10, e.g. 400MI-IZ, drives a tankcircuit Ll, C1, the inductor L1 of which is loosely inductively coupledwith another loop inductor L2, the two ends l6, 17 of which areconnected to respective arms 11 of a rabbit ears, half wave dipoleantenna. The arms 11 are mounted in an antenna mounting member 12secured to the top of a casing 13 in which the other componentsincluding a key operated switch 14 are housed. Due to the simplicity ofthe device, the casing 13 can be small, e.g. about 6 inches long, 3inches deep and 2 inches high, i.e. about the size of a small radioreceiver. It can readily be located in the area to be surveyed withoutoccupying any appreciate space. The components are operated from abattery 9, so that the device is independent of any power supply,although a battery charger can be included as an extra component. Sincethe device operates with radar frequency waves, its signals penetratewood and most other materials including thin walls; it can thus bemounted out of sight in a cupboard or other concealed location.

The center point 15 of the inductor L2 is connected to ground (casing)and hence to the member 12 which is of metal, preferably aluminum. Theconnections of the inductor ends 16, 17 to the antenna arms 11 are notdirect, but through capacitances C2 and C3. These series capacitances,which, at the frequency employed, exhibit very low impedances, are notseparate electrical components, but are built into the structure of theantenna mount, as is described in detail below.

In a similar fashion, the antenna arms 11 are physically mounted so asto have capacitances to ground represented by C4 and C5, these shuntcapacitances exhibiting relatively high impedances at the frequencyemployed. For example, capacitances C2, C3 could each be about 20 picofarad; and capacitances C4, C5 each about 0.1 pico farad. Thecapacitances C2 to C5 together with the inductor L2 form a network tunedto the oscillator frequency. While the tuning is not so sharp that thedoppler shifted frequencies are eliminated, this network does serve tofilter out extraneous signals of other frequencies, as may arise fromadjacent electrical equipment, e.g. motors, or from lightning.

Signals at 400MHZ reflected back to the antenna and shifted on frequencyby a moving object, e.g. a shift of the order of 0.1 to lOI-IZ, willbeat with the base frequency to produce the low difference frequency.This low frequency is recovered by demodulating the high frequency,using diodes D1 and D2 connected between points 16, 17 and a point 18,and providing full wave rectification. Since the tank circuit L1, C1presents a low impedance to the beat frequency, the loose couplingbetween inductors L1 and L2 is necessary. A capacitor C6 filters outhigh frequency signals, the beat frequency entering a low frequencyamplifier 19. The gain of the amplifier 19 can be varied to determinethe range of the device. Typically, with full gain, there has been foundto be no difficulty in obtaining a range radius of 20 feet around thedevice, using an oscillator of 10 milliwatts at low gain this radius canbe reduced to the order of about 5 feet. The height of the spaceeffectively covered by the radar field will be typically about half therange.

The output of the amplifier 19 energizes a trigger and timer circuit 20which causes continuous operation of a born 21 or other alarm mechanismfor a predetermined period, e.g. 2 minutes, whereupon the circuit resetsitself. When the device is tumed on by the owner using his key in switch14, his own subsequent movement away from the device will actuate thetrigger and timer circuit 20 to sound the born 21 for 2 minutes. Thisaction will provide a test that the device is functioning properly.After the 2 minutes have elapsed, the device will again become silent,when the trigger and timer circuit 20 resets itself, assuming that theowner has by now moved out of range and that no other moving objects arewithin range to reenergize the amplifier 19.

When an intruder or other moving object appears, the horn will sound for2 minutes, which may in itself be sufficient time to achieve the desiredresult, i.e. to scare away the intruder and/or to alert the householder,superintendent or night watchman. By switching off its horn after 2minutes, the device again becomes ready to detect a new intrusion. Ofcourse, if any moving intrusion persists, the horn will continue, the 2minutes duration of the horn being timed not from its initial actuationbut from the last energization of the circuit 20 by the amplifier 19.

If desired, as an optional feature, one or more other alarm mechanismsmay be connected to the device, e. g. a siren or bell external to thepremises, floodlights, or a police switchboard indicator. This otheralarm mechanism has been shown diagrammatically in FIG. 2 at 22 and canbe actuated by a trigger 23 energized through a delay device 24 from theamplifier 19. A short delay, e.g. 30 seconds or perhaps 1 minute,furnished by the device 24 will enable the owner to approach the alarmdevice to turn it on and off. Under these conditions he will operate thehorn 21, but he will not trigger the other alarm mechanism 22. In thecase of turning off, the key switch 14 will be opened, before the delayperiod has expired; in the case of turning on, the owner will have movedbeyond range, so that the amplifier 19 will no longer be ready toenergize the trigger 23 when the delay period expires.

The manner of securing each of the antenna arms 1 1 in the mountingmember 12 to achieve simultaneously the necessary structural andelectrical characteristics, is best seen from FIGS. and 6. A metal bolt30 extends through a number of parts and finally into a tapped cavity 31in the end of the arm 11. in sequence, starting at its head, thethreaded shaft of the bolt 30 passes through a dielectric insulatingwasher 32, having a boss portion 33; a metallic conductor in the form ofa lug 34 mounted on an end of the inductor L2 and having a hole 35 sizedto receive the boss portion 33; a dielectric insulating washer 36; ametallic washer 37; a nut 38; a dielectric insulating washer 39; thewall of the member 12; a further dielectric insulating washer 40; asecond nut 41; and finally the arm 11. The sides of the washers 39, 40facing the member 12 are recessed to form boss portions 42, 43 thatenter the hole in the member 12 and establish a firm structuralconnection therewith.

The first or series capacitances C2, C3 are each formed between the lug34 on the one hand, and the bolt 30 and metallic washer 37 on the otherhand, these latter parts being in direct metallic connection with thearm 11. These capacitances are comparatively large by virtue of thethinness of the dimension A of the dielectric washer 36, eg 4thousandths of an inch, and the thinness of the radial dimension B ofthe boss 33 of the dielectric washer 32, also about 4 thousandths.

The second or shunt capacitances C4, C5 are each formed between the nuts38, 41 and bolt 30 on the one hand, and the grounded mounting member 12on the other hand. These capacitances are comparatively small by virtueof the much greater values chosen for the dimensions C and D.

An important attribute of the system as described is an absence ofelements in the tuning circuits that are dependent on temperature andother variables such as humidity. This feature contributes significantlyto the operating stability of the system.

,As an alternative to using a bolt 30 with a head, a head-less bolt canbe used, threaded at least at both ends. The head will then be replacedby a nut that can be screwed onto one end of the bolt, the other endentering the tapped cavity 31.

lclaim:

1. An antenna mount comprising a. an antenna in the form of at least onemetallic arm,

b. a grounded metallic mounting member for supporting said arm,

c. an electrical conductor for capacitive coupling to said arm,

d. and means mechanically connecting said arm to the mounting member andto the conductor,

e. said connecting means comprising i. a metallic bolt having a head ornut at one end and a threaded shaft extending therefrom through a holein said conductor and then through a hole in said mounting member into atapped cavity in said arm,

ii. a pair of metallic nuts engaging the threaded shaft of the bolt, oneon each side of the mounting member,

iii. a metallic washer on the bolt between said conductor and the nutnearer the bolt head,

iv. first dielectric insulating means located between the bolt and thewasher on the one hand, and said conductor on the other hand, to form afirst capacitance between the antenna arm and said conductor,

v. second dielectric insulating means located between the two nuts andthe bolt shaft on the one hand, and the mounting member on the otherhand, to form a second capacitance between the antenna arm and themounting member,

vi. said first and second dielectric means being so dimensioned that thefirst capacitance is large compared to the second capacitance.

2. An antenna mount as claimed in claim 1, wherein said antennacomprises a pair of metallic arms arranged to form a half wave dipole,and wherein each said arm is mounted on said mounting member by arespective said connecting means (e).

3. The combination of an antenna mount according to claim 2, and radartransmission and receiving means comprising f. a first loop inductorhaving two ends each connected to a respective said electricalconductor, the center of said loop being grounded,

g. a second loop inductor loosely inductively coupled with said firstinductor, said second inductor having two ends,

h. a capacitor connected across the ends of the second inductor to forma tank circuit therewith, and

i. a radar frequency oscillator connected to said tank circuit,

an alarm means upon receipt of a signal at said low frequency.

5. The combination of claim 4, including timing means for interruptingsaid alarm means after a predetermined time interval.

6. The combination of claim 5, including further alarm means and delaymeans connected thereto and to said means (k) whereby to trigger saidfurther alarm means upon receipt of a signal at said low frequencysustained for a predetermined time interval.

1. An antenna mount comprising a. an antenna in the form of at least onemetallic arm, b. a grounded metallic mounting member for supporting saidarm, c. an electrical conductor for capacitive coupling to said arm, d.and means mechanically connecting said arm to the mounting member and tothe conductor, e. said connecting mEans comprising i. a metallic bolthaving a head or nut at one end and a threaded shaft extending therefromthrough a hole in said conductor and then through a hole in saidmounting member into a tapped cavity in said arm, ii. a pair of metallicnuts engaging the threaded shaft of the bolt, one on each side of themounting member, iii. a metallic washer on the bolt between saidconductor and the nut nearer the bolt head, iv. first dielectricinsulating means located between the bolt and the washer on the onehand, and said conductor on the other hand, to form a first capacitancebetween the antenna arm and said conductor, v. second dielectricinsulating means located between the two nuts and the bolt shaft on theone hand, and the mounting member on the other hand, to form a secondcapacitance between the antenna arm and the mounting member, vi. saidfirst and second dielectric means being so dimensioned that the firstcapacitance is large compared to the second capacitance.
 2. An antennamount as claimed in claim 1, wherein said antenna comprises a pair ofmetallic arms arranged to form a half wave dipole, and wherein each saidarm is mounted on said mounting member by a respective said connectingmeans (e).
 3. The combination of an antenna mount according to claim 2,and radar transmission and receiving means comprising f. a first loopinductor having two ends each connected to a respective said electricalconductor, the center of said loop being grounded, g. a second loopinductor loosely inductively coupled with said first inductor, saidsecond inductor having two ends, h. a capacitor connected across theends of the second inductor to form a tank circuit therewith, and i. aradar frequency oscillator connected to said tank circuit, j. the firstand second dielectric means associated with each respective antenna armbeing so dimensioned that said first and second capacitances form withthe antenna arms and with said first inductor a system tuned to saidradar frequency.
 4. The combination of claim 3, further including k.means connected to said first inductor for detecting a low frequencybeat between said radar frequency and a return frequency received by theantenna with doppler shift, and l. means connected to said means (k) fortriggering an alarm means upon receipt of a signal at said lowfrequency.
 5. The combination of claim 4, including timing means forinterrupting said alarm means after a predetermined time interval. 6.The combination of claim 5, including further alarm means and delaymeans connected thereto and to said means (k) whereby to trigger saidfurther alarm means upon receipt of a signal at said low frequencysustained for a predetermined time interval.